There is increasing awareness of the importance of ion channel accessory proteins in regulating heart rhythm. MinK and the related MiRPs are K+ channel regulators encoded by the KCNE family of genes. They can interact with the two Long QT-associated delayed rectifiers (HERG and KvLQT1 ), pacemaker channels (HCNs) and a variety of voltage gated K+ channels. Their importance is underscored by linkage to hereditary Long QT syndrome and single nucleotide polymorphisms that may sensitize patients to drug-induced arrhythmias. KCNEs each encode small integral membrane proteins with a single transmembrane segment suggesting that they interact with K+ channels in homologous fashion. Their sequence homology however, is fairly divergent with only scattered areas of conservation. Mutations have shown us several of the important sites in KCNE1 & 2 required for function. Controversy remains regarding structure-function relations, stoichiometry and specific protein-protein interactions in vivo. By studying the precise mechanisms of KCNE-channel interaction, a deeper understanding of LQTS, drug-induced and acquired arrhythmias may be gained. To this end we propose to: 1) Determine the specific regions of KvLQT1 and HERG that physically and functionally interact with minK and/or MiRPs. 2) Determine the relative preferences in partners between minK/MiRPs and HERG or KvLQTI. 3) Determine channel complex stoichiometry using epitope-tagged KCNEs and chimeras in biochemical and electrophysiological studies. 4) Analyze the co-expression pattern of minK/MiRPs against that of HERG and KvLQT1 and their association in cardiac myocytes.